CODEWITHSUNDEEP
unixtermios
Ben Turner
Ben Turner

Reputation: 765

Understanding UNIX termios VMIN and VTIME

I am currently working on a simple serial interface on a UNIX based device and cant find a definitive answer to the following:

I am currently trying to determine if a 'pure time read' (VMIN = 0, VTIME >0) will return half way through reading to n_bytes, as the timer is started when read is called, not when the first character is received.

For example, if I send a message to the device on the other end of the serial interface and I want a response I'd attempt the following (pseudo code):

m_tty.c_cc[VMIN] = 0;
m_tty.c_cc[VTIME] = 5;                              //i.e. > 0

write(myFileHandle, myData, sizeof(myData));
usleep(sizeof(myData) * 100);                      //assuming 100 us per char to Tx.
read(myFileHandle, myRxData, expectedMinNumBytes);

I am unclear as to whether read() would return if the first byte arrived just as the timer was about to expire, or if it would continue until 'expectedMinNumBytes' once the first is received?

Thanks for the help in advance!

Upvotes: 2

Views: 1589

Answers (1)

Jean-Baptiste Yunès
Jean-Baptiste Yunès

Reputation: 36431

This is a pure timed read. If there is available data, the read is immediately satisfied. If there is no data, the timer is started at the time read is called, and the read returns: either because the timer expires (returns 0) or a single byte is available.

Upvotes: 1

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